Chromatographic systems rely on the use of valves to allow reproducible sample introduction and various column switching schemes. Diaphragm-sealed type valves are commonly used in such systems. A typical diaphragm-sealed valve includes a valve cap having a plurality of ports opening on an interface. Each port is linked to a passage in the valve cap to which various analytical fitting and tubing may be connected. A diaphragm valve also includes a valve body having an interface opposite that of the valve cap. The diaphragm, generally made of a polymer material, is compressibly positioned between the opposite interfaces of the valve body and valve cap. A main recess is usually provided in the interface of the valve body, in which sits a matching recess in the diaphragm, allowing some clearance for fluid circulation between adjacent ports. This communication between ports can be stopped through the use of plungers slideably mounted in the valve body. Each plunger can press on the diaphragm between two adjacent ports, and therefore prevent fluid communication therebetween.
Examples of diaphragm-sealed valve can for example be seen in U.S. Pat. Nos. 3,111,849; 3,140,615; 3,198,018; 3,376,894; 3,387,496; 3,417,605; 3,439,542; 3,492,873; 3,545,491; 3,633,426; 4,112,766; 4,276,907; 4,333,500; 5,601,115; 6,202,698 and 7,216,528.
One of the problems of prior art diaphragm valves for gas chromatography is that the valve performance can vary greatly as a function of the operating temperature to which it is submitted. Variations in leak rate can be observed at moderate pressure, for example when the operating temperature is cycles such as is the case in temperature programming mode, or simply when the valve is operate continuously at temperature up to 350° or 400° C.
This performance variation is related to the fact that material dimensions of all the valve components, as well as the elasticity or the hardness of the polymer diaphragm, change with the temperature.
On the one hand, requirements for diaphragm-sealed valve design suitable for gas chromatography applications involve tight manufacturing tolerances for flatness, parallelism, in the surface finishes, and length of various components, especially the valve's plungers. Variations in plunger length will have a dramatic impact in the valve performance. The total effect of temperature induced dimension changes will generate leaks, particularly if the valve is subjected to rapid and large temperature variations that may generate distortions and continuous dimensions variation.
On the other hand, diaphragm variations in extreme conditions may be crucial as they can lead to permanent damage of the valve. In the prior art, the actuating pressure on the plungers, and the resulting force applied when a plunger is pushed against this diaphragm to interrupt fluid flow between two ports, does not vary with temperature. However, at high operating temperatures, the polymer diaphragm becomes softer and this same force may lead to permanent damage, by pushing away the material underneath the plunger area or simply by punching or leaving permanent marking on the diaphragm.
Referring to FIGS. 1, 1A and 1B (PRIOR ART), there is shown a typical pneumatic operating mechanism for the plungers of a diaphragm-sealed valve. Such a mechanism includes two sets of plungers, respectively designated as “normally closed and “normally opened” plungers, each set being attached to a corresponding piston. When a piston is moved into an upper position, it forces the corresponding plungers up against the diaphragm. Normally, the actuating pneumatic pressure of each piston is set to a value sufficient to generate the required force on the corresponding plungers to seal the diaphragm between ports, without over stressing the diaphragm.
Diaphragm-sealed valves are normally operated with the help of a three way electric solenoid valve. When the solenoid valve is powered ON (see FIG. 2 (PRIOR ART)), the actuating pressure is applied into actuating mechanism and when the solenoid valve is powered OFF (such as shown in FIG. 1B (PRIOR ART)), the pressure is evacuated from the actuator, normally to the atmosphere.
The stroke of the pistons is limited by the plungers pressing against the valve diaphragm. As a result, increasing the actuating pressure increases the force applied on the diaphragm by the plungers. Typical actuating pressure values for diaphragm-sealed valves range from 50 to 65 PSIG. If the available actuating pressure from the solenoid valve is higher, as is normally the case in a process plant environment where 125 PSIG are usually available, a pressure regulator must be use to decrease the supplied pressure to a safe level. This requires another piece of hardware and associated tubing inside the instrument, increasing the overall cost and necessitating a larger equipment inventory.
When a valve as shown in FIG. 1 is actuated at a typical actuating pressure, for example 60 PSIG, and the temperature is ramped up, another problem may appear, depending on the system configuration. The pressurized volume inside the valve is ramped up from ambient temperature to 300° C., generating a pressure rise of about 60 PSIG. This results in a final pressure inside the system of roughly about 120 PSIG. This generates an uncontrolled extra force on the diaphragm that contributes to diminished valve performance over the time. Since the resulting increase in force acting on the diaphragm coincides with operating conditions where the diaphragm material is softened by the high temperature, operating under these conditions will reduce dramatically the valve performance and lifetime.
In addition, it is known that slight variations in the manufacturing process or assembly may cause a plunger to be slanted or misaligned within the plunger, which can negatively effect the valve's operation.
There is therefore a need for a diaphragm-sealed type valve which alleviates at least some of the drawbacks of the prior art.